As well known, semiconductor device fabrication employs gases such as dopant gas, etchant gas, diffusion gas and purge gas used for manufacturing semiconductor devices. The semiconductor device fabrication requires that such gases have high purity. Further, the flow rates of the gases that determine characteristics of semiconductor devices should be precisely and rapidly controlled in semiconductor device fabrication.
As an example of techniques for controlling the flow rate of a gas in semiconductor device fabrication, a heat sensitive type mass flow rate controller operates as follows. A gas flowing though a flow passage of a body of the controller is distributed at a predetermined ratio via a bypass and then sent to a flow sensor. A thermal resistor of the flow sensor changes temperature by means of heat conduction according to the gas flow, a Wheatstone bridge detects the temperature change in the thermal resistor as a voltage change and outputs an electrical signal, and an amplifier amplifies the electrical signal from the Wheatstone bridge and inputs the amplified electrical signal into the controller. The controller compares the input electrical signal with a set point and opens or closes a control valve operated by a solenoid or thermal actuator based on the comparison results to control the flow rate of the gas.
However, the conventional heat sensitive type mass flow rate controller has a problem in that the flow rate of the gas is indirectly measured in such a manner that the temperature of the thermal resistor of the flow sensor is changed by heat capacity according to the gas flow and the temperature change in the thermal resistor is detected as the voltage change by the Wheatstone bridge, resulting in very low response. Further, the conventional heat sensitive type mass flow rate controller has problems in that it does not ensure linearity of the relationship between the flow rate and the electromotive force of the flow sensor throughout the entire range of flow rate of the gas, and its reliability is greatly deteriorated due to changes in the sensitivity of the flow sensor according to gas pressure. Moreover, the conventional heat sensitive type mass flow rate controller has a problem in that it is troublesome to change a compensation constant for use in the measurement of the flow rate according to the kind of gas.